Evidence for quasi-homogeneous evolution of massive stars

Rotation affects the evolution of massive stars. The paths followed by massive stars in the HR diagram are different depending on the initial rotation velocity. The transport of chemical species freshly produced in the core is also more efficient in rotating stars. In the case of very fast initial rotation, massive stars are expected to follow a peculiar evolution: instead of evolving off the main sequence by its cool side, stars become hotter and hotter and evolve blueward of the ZAMS. The reason is the very efficient chemical mixing: stars evolve quasi homogeneously. In this talk, we will present evidence for the existence of such an evolution from the analysis of early-type H-rich WN stars in the Magellanic Clouds and the Galaxy. The stellar parameters are determined by spectroscopic analysis and are used to place the stars in the HR diagram: they are on the hot side of the ZAMS. In addition, they still contain a large fraction of hydrogen in their atmosphere. This cannot be explained by standard evolutionary tracks. Quasi homogeneous evolution is able to reproduce all these properties. We will discuss the implications of these results in the context of the formation of long-soft gamma ray bursts and the collapsar model.